|Publication number||US7573923 B2|
|Application number||US 11/775,409|
|Publication date||Aug 11, 2009|
|Filing date||Jul 10, 2007|
|Priority date||Jul 10, 2007|
|Also published as||US20090016389|
|Publication number||11775409, 775409, US 7573923 B2, US 7573923B2, US-B2-7573923, US7573923 B2, US7573923B2|
|Original Assignee||Applied Optoelectronics, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (4), Referenced by (8), Classifications (7), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to modulated optical systems and more particularly, to a laser drive circuit and method that provides high limit clipping corresponding to low limit clipping in a laser to reduce even order distortion in modulated optical systems.
A laser may be used as an optical transmitter that transmits light at a given wavelength. The power (i.e., amplitude) of the laser light may be modulated by corresponding modulation of the power used to drive the laser. In a directly-modulated electrically pumped semiconductor laser such as a laser diode, the electrical current that drives or pumps the laser is modulated. The relationship between the light output and the input current for such a laser may be represented using a transfer curve or L-I (light-current) curve. The set point of the L-I curve may be selected so as to maximize the linearity of the laser output in response to the modulation, within the expected range of operation of the output produced by the laser. Although the laser output may be generally linear along a significant portion of the L-I curve, the light output may attain a zero-power level when the input current falls below a threshold current level, which results in an effect known as clipping.
In a communications system where multiple channels are transmitted, such as a CATV system, multiple analog signals corresponding to the multiple channels may be combined into a wide-band multichannel RF signal, which drives a laser to produce a multichannel modulated optical signal. The multiple analog signals may include multiple modulated analog carriers that may be combined, for example, using frequency division multiplexing techniques. One or more digital signals modulated using digital modulation, such as quadrature amplitude modulated (QAM), may also be combined with the modulated analog carrier signals, for example, using subcarrier multiplexing (SCM) techniques. In some systems, for example, as many as 110 channels may be transmitted over a frequency range of about 50 MHz to 750 MHz.
Because the modulation may carry several channels of information at different frequencies, there may be a very large swing of the input drive current in either direction. When many signals are summed and are randomly distributed in both frequency and phase, the ratio of peak-to-average voltage rarely exceeds 14 dB (though with occasional higher peaks). In a CATV system, however, the downstream spectrum is not random. Peak voltage conditions may occur, for example, when a large number of carriers are harmonics of a common root frequency and the carrier phases are aligned. In that case, the time domain waveform can resemble a string of impulses spaced by a time interval equal to the period of the common root frequency. As a result of this occasionally occurring peak voltage (and thus peak drive current) condition, the laser may be driven into hard limiting, causing clipping, when a sufficient number of carriers are in phase alignment. This is particularly true in the case of directly modulated laser diodes, as described above, where a sharp knee occurs in the transfer function below which the light output reaches a zero-power level.
In other words, there will be clipping when the instantaneous sum of various signals causes the drive current to swing too far in the “downward” direction and below the threshold current that turns on the laser. When such clipping occurs, intermodulation products (i.e., clipping-induced distortion) and noise may be generated, which may result in bit errors in the optical output of the laser. When polarity of a sine wave is clipped, both even and odd order distortion products may be generated, although second order distortion products are the largest.
The even-order distortion includes composite second order (CSO) distortion products, i.e. distortion products of the type 2f1, 2f2, f2−f1, and f2+f1. In particular, CSO is a second-order distortion that combines signals at frequencies A and B, as AħB. The odd-order distortion includes composite triple beat (CTB) distortion. CTB (also known as C/CTB) is a third-order distortion product that combines signals at frequencies A, B, and C as A+B−C. For optical transmitters modulated externally with Mach-Zehnder external modulators, the nonlinearities are symmetrical and the limiting is “softer,” resulting in less clipping and primarily odd-order distortion. For directly-modulated DFB lasers, however, both CSO and CTB will show an increase when such clipping happens with sufficient frequency to be statistically significant.
These and other features and advantages will be better understood by reading the following detailed description, taken together with the drawings wherein:
The RF signal 132 may be a multichannel RF signal including multiple superimposed modulated analog carriers at different frequencies. The multiple modulated analog carriers may be modulated using modulation techniques known to those skilled in the art, such as amplitude modulation, and may be combined using multiplexing techniques known to those skilled in the art, such as frequency division multiplexing. The multichannel RF signal 132 may also include one or more digital signals modulated using digital modulation, such as quadrature amplitude modulation (QAM). The resulting multichannel RF signal 132 occupies a bandwidth across the range of frequencies of the multiple modulated carriers. Those skilled in the art will recognize that various modulation and multiplexing techniques may be used to generate the multichannel RF signal.
In one embodiment, the multichannel RF source 130 may include headend equipment in a CATV system and the multichannel RF signal 132 may be a downstream CATV signal. Examples of downstream multichannel CATV signals include 77 channels transmitted over a frequency range of about 50 MHz to 550 MHz and 110 channels transmitted over a frequency range of about 50 MHz to 750 MHz. Each channel in a downstream multichannel CATV signal may include a video carrier, a color subcarrier and an audio carrier. Other types of signals and frequency ranges may also be transmitted. The bias current source 140 may include any type of bias current source and/or bias current control used to bias a laser diode, for example, in an optical transmitter.
In the exemplary embodiment, each channel in the multichannel RF signal 132 may be driven or modulated up to a certain optical modulation index (OMI) depending upon a desired channel-to-noise ratio (CNR). In one embodiment, the OMI of at least some of the channels may be in a range of about 2% to 5%. When multiple modulated carriers of the multichannel RF signal 106 align in phase, the sum of the voltage of the aligned carriers may result in a peak voltage condition. When the optical modulation index (OMI) of each channel exceeds a certain level (e.g., exceeding about 3% OMI per channel), the peak voltage condition may result in a higher occurrence of negative voltage spikes or peaks that cause the laser input current to fall below a threshold current of the laser 110, resulting in clipping.
The laser 110 may include a semiconductor laser, such as a laser diode, having an L-I curve with a sharp knee or point at which the light output reaches a zero level when the input current falls below the threshold current (Ith). Referring to
To provide high limit clipping, the input drive current as represented by drive current waveform 310 is clamped at the high limit current (Ih) resulting in a clamped peak 314, for example, when the combined the RF signal and bias current exceeds the high limit current (Ih). Thus, when the input drive current is clamped at the high limit current (Ih), the optical output abruptly stops as represented by the clipped positive peak 324 in the optical output waveform 320. As a result of the high limit clipping, the transfer curve 300 effectively has a flat portion 304 corresponding to the high limit clipping.
The high limit current (Ih) may be selected such that the high limit clipping is substantially symmetrical to the low limit clipping that occurs when the drive current falls below the threshold current (Ith) of the laser. In other words, the clipped negative peak 322 and the clipped positive peak 324 extend by a substantially equal amount below and above some baseline or unmodulated level 326 corresponding to the bias current (Ibias). The high limit current (Ih) is thus based on the threshold current (Ith) of the laser and the bias current (Ibias). The degree to which the high limit clipping and the low limit clipping are symmetrical may depend upon the clipping distortion that can be tolerated in a particular system, as discussed below.
By making the high limit clipping substantially symmetrical with the low limit clipping, composite second order (CSO) distortion may be reduced because CSO distortion is even-order and symmetrical. Although composite triple beat (CTB) distortion may be increased by the high limit clipping, the magnitude of the CTB distortion is generally smaller than CSO distortion and may be tolerated. Thus, CSO distortion may be reduced at the cost of increasing CTB distortion, which results in a “softer” limiting because the non-linearities are more symmetrical. The laser drive circuit and method, consistent with embodiments of the present invention, thus produce the unexpected and unpredictable result of reducing the effects of clipping distortion by inducing high limit clipping. The laser drive circuit and method described herein may be especially useful in applications where CSO distortion is a problem but CTB distortion can be tolerated.
According to this embodiment, the current clamp 420 includes a zener diode 426 and another circuit element 428 such as a resistor. When the input current (I1) to the current clamp 420 exceeds the high limit current (Ih), the zener diode 426 drains away the excess current so that the clamped current (I2) at the output 424 of the current clamp 420 does not exceed the high limit current (Ih), thereby causing high limit clipping of the optical output of the laser diode 410. The circuit element 428 and the zener diode 426 may be selected and configured based on the desired high limit current (Ih). The zener diode 426, for example, may have a rated breakdown voltage or zener voltage that prevents current through the current clamp 420 from exceeding the desired high limit current (Ih). As mentioned above, the desired high limit current (Ih) may be based on the threshold current (Ith) and bias current (Ibias) of the laser 410 such that the high limit clipping caused by the current clamp 420 is substantially symmetrical with the low limit clipping that occurs in the laser diode 410.
Accordingly, a laser drive circuit, consistent with embodiments of the present invention, clamps a drive current to a laser to provide high limit clipping corresponding to the natural low limit clipping that occurs in the laser. The high limit clipping may be substantially symmetrical to the low limit clipping to reduce CSO distortion.
Consistent with one embodiment, a laser drive circuit includes a laser including a drive current input configured to receive a drive current and an optical output configured to provide a modulated optical signal in response to the drive current. The laser has a threshold current such that low limit clipping occurs when the drive current to the laser falls below the threshold current. The laser drive circuit also includes a current clamp including an input configured to receive at least a RF signal and an output configured to provide a clamped current to the drive current input of the laser diode. The current clamp is configured to clamp the current of the RF signal to a high limit current such that the clamped current provided to the drive current input of the laser diode provides high limit clipping corresponding to the low limit clipping that occurs in the laser.
Consistent with another embodiment, a method includes providing a RF signal including a plurality of positive spikes and a plurality of negative spikes; clamping a current of at least the RF signal to a high limit current to provide a clamped drive current; providing the clamped drive current to an input of a laser such that the clamped drive current provides high limit clipping corresponding to low limit clipping that occurs in the laser; and providing a modulated optical signal from the laser.
While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.
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|U.S. Classification||372/38.07, 372/38.02, 372/38.1|
|Cooperative Classification||H01S5/0427, H01S5/042|
|Jul 11, 2007||AS||Assignment|
Owner name: APPLIED OPTOELECTRONICS, INC., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:ZHENG, JUN;REEL/FRAME:019540/0569
Effective date: 20070709
|Feb 25, 2009||AS||Assignment|
Owner name: UNITED COMMERCIAL BANK, CALIFORNIA
Free format text: SECURITY AGREEMENT;ASSIGNOR:APPLIED OPTOELECTRONICS, INC.;REEL/FRAME:022299/0966
Effective date: 20070906
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Owner name: EAST WEST BANK, CALIFORNIA
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